The present invention generally relates to scanning line interpolation circuits and, more particularly, to a scanning line interpolation circuit wherein a frequency band of a sequential scanning auxiliary signal (V-T helper signal) is limited at the transmitter side and interlaced scanning lines not transmitted are reproduced by interpolation on the basis of television signals including the sequential scanning auxiliary signal and a main signal which are both transmitted.
In an existing television system (in an NTSC system), a picture is transmitted with one frame divided into two fields by interlaced scanning. A conventional TV receiver has had such a problem that, since display is carried out based on interlaced scanning, the roughness or flickering of a reproduced picture caused by every line scanning results in deterioration of the picture.
In an attempt to reduce the deterioration of the picture quality, there have been suggested an EDTV system which maintains full compatibility with an existing television system and an IDTV system in which an existing television signal is processed or subjected to an interlace-to-sequential-scanning conversion at the receiver side to provide highly fine display. In the latter case, a motion in a picture image is detected and scanning-line interpolation processing is performed on the basis of a processing parameter indicative of the detected motion (for example, refer to JP-A-58-130685).
Since the transmitted signal is an interlace signal, however, this method is restricted, and it is in principle, impossible to detect some motion. For example, since the system cannot distinguish a picture moving at a speed just equal to a frame period (1/30 seconds) from a still picture, the system performs an interpolation processing over the actually moving picture in a still picture mode, thus resulting in picture quality being remarkably deteriorated. Further, in a part judged as a moving picture, since a scanning line which is not transmitted is generated by interpolating upper and lower scanning lines in an identical field, the system disadvantageously cannot reproduce a picture having a high vertical frequency (e.g., fine horizontal streak).
For the purpose of avoiding the above disadvantage, there has been considered a system that an ordinary transmission signal (which will be abbreviated to the main scanning line signal, hereinafter) is generated through sequential-to-interlace scanning conversion with use of a sequential scanning camera on the transmitter side, and a signal (which will be abbreviated to the sequential scanning auxiliary signal, hereinafter) for assisting the scanning line interpolation at the receiver side is transmitted together with the sequential scanning auxiliary signal from the transmitter side to the receiver side. In an advanced compatible TV (ACTV) system suggested by the David Sarnoff Research Center) DSR in the U.S., for example, a difference signal (field difference signal) corresponding to a difference between a scanning line of a first field not transmitted due to interlaced scanning and a scanning line at the corresponding position in a field before or after the first field by one field is transmitted as a sequential scanning auxiliary signal (V-T helper signal).
Referring to FIG. 2, there is schematically shown an exemplary arrangement of a scanning line auxiliary circuit used in the prior art ACTV receiver (as disclosed in IEEE Transactions on Consumer Electronics. Vol. 34, No. 1, Feb. 1988), wherein an input signal transmitted on a multiplex basis is separated at a separator 1 into a main scanning line signal (interlace signal) and a sequential scanning auxiliary signal (V-T helper signal) and then subjected at an auxiliary scanning line reproducer 102 to an interpolation to reproduce a scanning line not transmitted. FIGS. 3A to 3C illustrate how to reproduce a scanning line not transmitted. More specifically, FIG. 3A illustrates a positional relation between main scanning line signals to be transmitted and FIG. 3B shows a positional relation between sequential scanning auxiliary signals. In FIG. 3A, reference symbols A and B denote scanning lines located horizontally immediately before and after a scanning line X not transmitted and at the same position on the line X. In the above ACTV system mentioned above, as shown in FIG. 3B, a signal (X-(A+B)/2) is transmitted as a sequential scanning auxiliary signal Y. On the receiver side, as shown in FIG. 3C, the scanning line X (=Y+(A+B)/2) not transmitter is reproduced on the basis of the transmitted scanning lines A and B and auxiliary signal Y.
FIGS. 4A to 4C show another method of reproducing a scanning line not transmitted. More specifically, FIG. 4A shows a positional relation between main scanning line signals to be transmitted and FIG. 4B shows a positional relation between sequential scanning auxiliary signals. In FIG. 4A, reference symbols C and D denote scanning lines located vertically immediately before and after a scanning line X not transmitted and in the same field. As shown in FIG. 4B, a signal (X-(C+D)/2) is transmitted as a sequential scanning auxiliary signal Y. In the receiver side, on the other hand, as shown in FIG. 4C, the scanning line X (=Y+(C+D)/2) not transmitted is reproduced on the basis of the transmitted scanning lines C and D and auxiliary signal Y.
Accordingly, with respect to a frequency band used to transmit the sequential scanning auxiliary signal, the original sequential scanning line signal can be reproduced on the receiver side without any deterioration.
If the sequential scanning auxiliary signal corresponding the same frequency band width as the main scanning line signal can be transmitted as by utilizing a plurality of channels, then there occurs no problem. However, when the number of idle channels is small, both the main scanning line signal and sequential scanning auxiliary signal must be transmitted on one channel.
In such an ACTV system as mentioned above, as shown in FIG. 5A, a carrier orthogonal (different in phase by 90 degrees from) to a video carrier for modulation of the main scanning line signal is used to modulate the sequential scanning auxiliary signal for one-channel transmission. This method, however, permits the transmission of the sequential scanning auxiliary signal having a frequency band of only about 1 MHz.
There is another method of masking parts of a screen with black belts and unnoticeably multiplexing the auxiliary signal in the mask parts as shown in FIG. 5B. This method, however, essentially disables enlargement of the mask parts and thus requires the auxiliary signal to be transmitted with its frequency band limited to about 1/3 to 1/55.
In the prior art ACTV system of FIG. 2, the sequential scanning auxiliary signal with its limited frequency band of 750 kHz and the main scanning line signal with its full frequency band are subjected to a scanning line interpolation in such a manner as shown in FIG. 3. For this reason, for signals having frequency above 750 kHz, an interframe average (Y=(A+B)/2) is output as an interpolated scanning line so that the time resolution drops and the moving picture blurs undesirably. When the scanning line interpolation is carried out in such a manner as shown in FIG. 4, an inter-scanning line average (Y=(C+D)/2) is output for the high frequency zone of the interpolated scanning line, thus resulting in that the vertical resolution drops.